Abstract
The spatial distribution of radiopharmaceuticals that emit short-range high linear-energy-transfer electrons greatly affects absorbed dose and biological effectiveness. The purpose of this study was to investigate the effect of heterogeneous radionuclide distribution on tumor control probability (TCP) in a micrometastasis model. Methods: The cancer cell lines MDA-MB-468, SQ20B, and 231-H2N were grown as spheroids to represent micrometastases. The intracellular distribution of a representative radiopeptide (111In-labeled epidermal growth factor) and radioimmunotherapeutic (111In-labeled trastuzumab) was determined in cell internalization experiments. The intratumoral distribution was evaluated by microautoradiography of spheroids. γH2AX staining was performed on spheroid sections to correlate DNA damage with radionuclide distribution. Experimental surviving fractions were obtained using clonogenic assays. A random close-packed algorithm, which models the random packing behavior of cells and reflects variation in the radii of cells and nuclei, was used to simulate 3-dimensional spheroids. Calculated survival fractions were generated using an iterative modeling method based on Monte Carlo–determined absorbed dose with the PENELOPE code and were compared with experimental surviving fraction. Radiobiologic parameters deduced from experimental results and Monte Carlo simulations were used to predict the TCP for a 3-dimensional spheroid model. Results: Calculated survival fractions agreed well with experimental data, particularly when an increased value for relative biological effectiveness was applied to self-dose deposited by sources located in the nucleus and when radiobiologic parameters were adjusted to account for dose protraction. Only in MDA-MB-468 spheroids treated with 111In-epidermal growth factor was a TCP of more than 0.5 achieved, indicating that for this cell type the radiopeptide would be curative when targeting micrometastases. This ability is attributed to the relative radiosensitivity of MDA-MB-468 cells, high nuclear uptake of the radiopeptide, and uniform distribution of radioactivity throughout the spheroid. Conclusion: It is imperative to include biologic endpoints when evaluating the distribution of radionuclides in models emulating micrometastatic disease. The spatial distribution of radioactivity is a clear determinant of biological effect and TCP as demonstrated in this study.
Highlights
The predicted relative biological effectiveness of targeted radionuclide therapy is often extrapolated from that of externalbeam radiotherapy
It is postulated that Auger electron (AE)–emitting radionuclides that bind or intercalate into DNA are ideal for targeted radionuclide therapy of single cells, limited-volume disseminated cancer, and micrometastases [1,2]
Cell and nucleus radii used for Monte Carlo (MC) simulations were 9.45 6 1.71 mm and 6.65 6 1.30 mm, respectively, for MDA-MB-468, 10.61 6 1.24 mm and 8.10 6 1.39 mm, respectively, for SQ20B; and 11.21 6 2.59 mm and 7.09 6 1.44 mm, respectively, for 231-H2N (Figs. 2D–2F)
Summary
The predicted relative biological effectiveness of targeted radionuclide therapy is often extrapolated from that of externalbeam radiotherapy. It is postulated that Auger electron (AE)–emitting radionuclides that bind or intercalate into DNA are ideal for targeted radionuclide therapy of single cells, limited-volume disseminated cancer, and micrometastases [1,2]. This hypothesis is based on the high linear-energy transfer (4–26 keV/mm) of lowenergy AEs, which deposit energy within a few cubic nanometers of the decay site [1,3]. The localized absorption of electrons results in complex irreparable DNA damage [4] This feature endows AE-emitting targeted radionuclide therapy agents with an enhanced relative biological effectiveness when compared with a- or b-emitting radionuclides.
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
